This invention relates to the preparation of phthalic anhydride by oxidation of o-xylol or naphthalene, and in particular, to a catalyst for use in the preparation of phthalic anhydride.
In conventional processes for preparing phthalic anhydride by oxidation of ortho-xylol or naphthalene in the vapor phase with an oxygen containing gas, supported catalysts are employed which mostly comprise, as the active portion, vanadium pentoxide and titanium dioxide, with minor additives such as stabilizers, promoters, etc., and as the support or carrier, particles of a non-reactive material, such as silicates or alumina, which supply the surface area, strength and stability required for the catalyst.
The catalysts for phthalic anhydride used in the past comprised supports or carriers preferably in the forms of balls, saddle-pieces, or cylinders.
For the active portion of the catalysts for preparing phthalic anhydride, it has been common practice to use titanium dioxide with pores so distributed as to have pore diameters mostly in the 0.10 to 0.50 microns range.
While such known catalysts exhibit a satisfactory catalytic activity in the preparation of phthalic anhydride, they are not entirely devoid of shortcomings, as connected with their potentiality and selectivity levels.
Furthermore, such prior catalysts require in the phthalic anhydride preparation process, which is notoriously a fixed bed one, a high oxidative gas to o-xylol or naphthalene ration, and hence the circulation of large masses of an oxidative gas (air) through the process. This brings about as obvious disadvantages the obtainment of a final product with a reduced concentration of phthalic anhydride (which is accordingly separated mostly as a solid by sublimation), the need to have heavy duty equipment available, and finally a considerably high energy expenditure due both to the need of recovering the sublimed phthalic anhydride and to the large amount of air being circulated.